U.S. patent number 6,448,924 [Application Number 09/417,149] was granted by the patent office on 2002-09-10 for microwave blade tracker.
This patent grant is currently assigned to Smiths Aerospace, Inc.. Invention is credited to John W. Hafer, Jr..
United States Patent |
6,448,924 |
Hafer, Jr. |
September 10, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Microwave blade tracker
Abstract
Blade tracker apparatus for tracking rotating helicopter blades
includes an antenna which transmits a signal beam in the direction
of the rotating blade. The impedance of the antenna changes as one
of the blades enters the transmission beam and the amount by which
the impedance changes varies as a function of the physical distance
between the antenna and the blade. The varying impedance caused by
a blade passing the antenna will amplitude modulate the transmitted
signal and provide an indication of the distance of each of the
several blades of a rotor as each of the blades passes through the
field of the antenna.
Inventors: |
Hafer, Jr.; John W. (San Diego,
CA) |
Assignee: |
Smiths Aerospace, Inc. (Grand
Rapids, MI)
|
Family
ID: |
23652777 |
Appl.
No.: |
09/417,149 |
Filed: |
October 12, 1999 |
Current U.S.
Class: |
342/28; 342/118;
342/195; 342/27 |
Current CPC
Class: |
B64C
27/008 (20130101) |
Current International
Class: |
B64C
27/00 (20060101); B64D 45/00 (20060101); G01S
013/56 (); G01S 013/08 (); G01S 013/04 () |
Field of
Search: |
;342/27,28,61,73,74,81,82,83,84,88,89,94,98,99,102,118,159,165,173,174,175,195
;340/561 ;343/7MS |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
1116748 |
|
Jun 1968 |
|
GB |
|
1143339 |
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Feb 1969 |
|
GB |
|
2055269 |
|
Feb 1981 |
|
GB |
|
Other References
Nagy, P.B. amd Greguss, P., Helicopter Blade Tracking by Laser
Light, Optics and Laser Technology, Dec. 1982, pp. 299-302, 1982
Butterworth & Co (Publishers) Ltd. .
Nagy, Peter B., Measurement of Conic Running with Laser Light,
Meres es Automatika 30, 1982, No. 3, pp. 102-106, (In Hungarian
Without English Translation)..
|
Primary Examiner: Gregory; Bernarr E.
Attorney, Agent or Firm: Varnum, Riddering, Schmidt &
Howlett LLP
Claims
The embodiment of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. Apparatus for detecting movement within a predefined spatial
area of a selected blade of a rotating unit having a plurality of
blades, said apparatus comprising: an antenna disposed at a fixed
position adjacent said spatial area and having a predetermined
antenna impedance; a signal source connected to said antenna for
transmitting a radio frequency signal to said antenna, said antenna
responsive to said radio frequency signal to radiate energy in said
predefined spatial area; detection circuitry connected to said
antenna for detecting deviation in said antenna impedance from said
predetermined antenna impedance resulting from the movement of said
selected blade in said spatial area; circuitry operating in
synchronism with said rotating unit for generating a periodic
signal; and circuitry responsive to said deviations in said antenna
impedance and to said periodic signal for generating output signals
indicative of distance of said selected blade from said
antenna.
2. The apparatus in accordance with claim 1, wherein said antennae
comprises a substantially flat metallic layer disposed on one side
of a substrate.
3. The apparatus in accordance with claim 2, and further comprising
a ground plane formed of a metallic layer disposed on an other side
of said substrate, opposite said one side.
4. Apparatus for determining distance of a moving object from a
fixed position, the apparatus comprising: a first and second
antennas disposed at said fixed position and each having a
predetermined antenna impedance; a signal source connected to said
first and second antennas for transmitting a radio frequency signal
to said antennas causing said antennas to radiate energy in first
and second predefined antenna fields; and circuitry connected to
said antennas for detecting changes in said predefined antenna
impedance of said first and second antennas resulting from the
presence of said object in said first and second predefined antenna
fields, respectively, and for generating an output signal
indicative of distance of said object from said fixed position.
5. Tracking apparatus for determining a path of travel of each of a
plurality of blades of a moving rotor, the apparatus comprising: a
first antenna disposed at a first fixed position adjacent said
rotor and a second antenna disposed at a second fixed position
adjacent said rotor, each of said antennas having a predefined
antenna impedance; a signal source connected to said first and said
second antennas for transmitting radio frequency signals to said
antennas, said antennas responsive to said radio frequency signals
to radiate energy in predefined antenna fields adjacent said rotor;
and circuitry connected to said antennas for detecting changes in
said predefined antenna impedance of said first and second antennas
resulting from the presence of one of said blades in said
predefined antenna fields and for generating an output signal
indicative of distance from said antenna of a propeller blade
traversing said antenna fields.
6. The tracking apparatus in accordance with claim 5, wherein said
rotor and said antennas are mounted on an aircraft and said
circuitry is operative to generate a separate output signal for
each of said rotor blades indicative of distance of each of said
rotor blades from said antennas, whereby an indication of tracking
of said rotor blades relative to each other is generated.
7. Tracking apparatus, for determining a path of travel of each of
a plurality of blades of a moving rotor, the apparatus comprising:
an antenna having a predefined antenna impedance and disposed
adjacent said rotor; a signal source connected to said antenna for
transmitting radio frequency signals to said antenna, said antennas
responsive to said radio frequency signals to radiate energy in a
predefined antenna field adjacent said rotor; and circuitry
connected to said antenna for detecting changes in said predefined
antenna impedance resulting from the presence of one of said blades
in said predefined antenna field and for generating an output
signals indicative of distance from said antenna of a propeller
blade traversing said antenna fields.
8. The tracking apparatus in accordance with claim 7, wherein said
antenna comprises first and second metallic layers disposed
side-by-side and spaced apart on a printed circuit board disposed
on a helicopter provided with a rotor having a plurality of rotor
blades moving along a predefined path of travel, said circuit board
disposed in alignment with said path of travel.
9. The antenna in accordance with claim 8, herein the circuit board
is disposed within a radome.
10. Apparatus for determining the path of travel of each of a
plurality blades of a helicopter rotor moving a path of travel, the
apparatus comprising: an antenna disposed adjacent said path of
travel and having a predetermined antenna impedance; a signal
source connected to said antenna for transmitting a radio frequency
signal to said antenna; said antenna responsive to said radio
frequency signal to radiate energy in a predefined antenna field
extending over said path of travel; and detection circuitry
connected to the antenna for detecting changes in said predefined
antenna impedance resulting from the presence of one of said
helicopter blades in said path of travel; and circuitry responsive
to said detected changes to compute a distance of said one of said
helicopter blades from said antenna.
11. The apparatus in accordance with claim 10, wherein said
detection circuitry detects changes in phase and amplitude of said
signal and said circuitry responsive to said detected changes in
phase and amplitude comprises circuitry for computing said
distance.
12. Apparatus for determining separation of moving objects
traveling along a predefined path of travel in a predefined spatial
area, the apparatus comprising: a first antenna having a
predetermined antenna impedance and disposed at a first position
adjacent said path; a second antenna having a predetermined antenna
impedance and disposed at a second position adjacent said path and
spaced apart from said first position by a predefined distance; a
signal source connected to said first and second antennas for
transmitting radio frequency signals to said antennas; said first
and second antennas responsive to said radio frequency signals to
radiate energy in said predefined spatial area; detection circuitry
connected to said first and second antennas for detecting
deviations in impedance of said first antenna from said
predetermined antenna impedance of said first antenna and
deviations in impedance of said second antenna from said
predetermined impedance of said second antenna resulting from the
movement of said object along said predefined path and for
generating first and second output signals representative of said
deviations in impedance of said first and second antennas,
respectively; and circuitry connected to said detection circuitry
and responsive to said first and second output, signals to generate
timing signals representative of time of entry and time of exit of
said objects.
13. The apparatus in accordance with claim 12 and further
comprising computational circuitry responsive to said timing
signals for determining spatial separations between objects of said
plurality of objects.
14. The apparatus in accordance with claim 12 and further
comprising timing apparatus responsive to said timing signals.
15. A sensing apparatus for determining the vibration
characteristics of a series of fan blades traveling in a
predetermined spatial area, said apparatus comprising: an antenna
disposed at a fixed position adjacent said spatial area and having
a known intrinsic antenna impedance; a signal source connected to
said antenna for transmitting a radio frequency signal to said
antenna, said antenna responsive to said radio frequency signal to
radiate energy in said spatial area; detection circuitry connected
to said antenna for detecting deviation in said antenna impedance
from said intrinsic antenna impedance resulting from movement of
each of said fan blades in said spatial area and for generating a
plurality of distance output signals, each indicative of distance
of one of said blades from a predetermined position.
16. The circuitry in accordance with claim 15 wherein the detection
circuitry further comprises circuitry responsive to said distance
output signals for comparing said distance output signals and for
identifying distance output signals differing from a predefined
signal value by more than specified amount, whereby excessive
vibration of a fan blade of said plurality of fan blades may be
detected.
17. The circuitry in accordance with claim 16 wherein each of said
distance output signals has a predefined distance output signal
value and the predefined output signal value is an average value of
said distance output signal values.
18. Apparatus for detecting movement of an object in a predefined
spatial area, said apparatus comprising: an antenna comprising a
substantially flat metallic layer and disposed at a fixed position
adjacent said spatial area and having a predetermined antenna
impedance; a signal source connected to said antenna for
transmitting a radio frequency signal to said antenna, said antenna
responsive to said radio frequency signal to radiate energy in said
predefined spatial area; and detection circuitry connected to said
antenna for detecting deviation in said antenna impedance from said
predetermined antenna impedance resulting from the movement of said
object in said spatial area.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to method and apparatus for sensing the
presence of a moving object and for generating outputs indicative
of the path of travel of moving objects and more particularly to
method and apparatus for sensing the presence and/or relative
position of a rotating fan blade or helicopter blade.
2. Background Art
It is well known that vibration resulting from rotating objects are
often detrimental. Both military and commercial rotorcraft
operators wage a continual battle against vibration, both to extend
the life of the rotorcraft and to enhance the comfort of the crew
and passengers. The rotor itself is a principal contributor to
vibration. However, rotor vibration can be reduced through proper
balancing of the rotor blades. Various devices have been developed
and sold over the last several decades to provide the information
required to accurately and efficiently balance a rotor. The more
accurate and efficient rotor track and balance systems incorporate
a blade tracking device to measure blade height and lead/lag
between the respective blades of the rotor. Measurements of these
blade parameters may be used in a balancing algorithm to generate
indicia representing recommended changes in blade adjustments in
order to minimize vibration. In addition, blade tracking
information is required in an over-all rotorcraft tuning algorithm
and for the successful completion of various required maintenance
actions.
In order to reduce damage to the engine, transmission, and airframe
caused by the vibration of helicopter blades, it is important that
all blades of a rotor travel in the same plane and that all blades
rotate with a fixed angular separation. To detect that the several
blades of a helicopter rotor or the like travel in the same plane,
it is a common practice to measure the distant of each of the
blades from a fixed point on the craft when the blades are in
predefined position relative to the fixed point. Prior art blade
tracking systems primarily use strobe lights and optical sensors
which measure light reflected from the rotating blades to determine
the relative position of the moving blades. Some blade tracking
systems have used electrostatic sensing probes or radiation,
whereby a lens focuses a beam of radiation in a plane and the
sensing probes detect the presence of an object as it passes
through the radiation beam. Other prior art systems employ an
oscillator connected to a large capacitive element positioned
adjacent the plane of rotation of the blades of a rotor. In such a
system, blade tracking measurements are derived from frequency
modulations, resulting from a change in capacitance, and displayed
on an oscilloscope.
A general problem with tracking devices known from the prior art is
the lack of accuracy of the generated output signals. Prior art
tracking devices using electrostatic probes are prone to error due
to undefined static sources and electrostatic changes in the
atmosphere which may influence accuracy of any static electricity
detectors. Most earlier blade tracking systems, including those
systems relying on change in capacitance, typically measure the
position of the several blades of a propeller while the craft is on
the ground. However, it is highly desirable that improper tracking
be detected in flight, since blade dynamics vary based on engine
speed and load. Commonly used tracking devices which rely on
reflected optical signals are generally inaccurate because of
interference due to ambient light and cannot be used at night
unless the blades are artificially illuminated. In addition,
reflective tape is commonly required on the blades. Furthermore,
optical tracking systems tend to be sensitive to color. On
helicopter rotors, color may vary from blade to blade and is often
white or shades of gray. Optical sensors have been known to be
incorrectly triggered by rotor shadows and to be sensitive to
varying ultraviolet light levels across the zenith.
Known optical devices detect the passage of rotor blades through
their field of view and generate pulse edges as each rotor blade
enters and leave the optical sensing region. The path of the
rotating blade is detected by using two such optical devices, such
that their corresponding optical sensors are separated by a known
angle. Precise time measurements of the pulse edges detected at the
two optical sensing regions, coupled with rotor speed and
installation parameters, allow blade height to be calculated.
Synchronized timer circuits are used to identify the time that each
blade enters or leaves the optical sensing region of each sensor. A
once-per-revolution timing pulse is simultaneously measured to
provide both rotational phase and rate information. Specifically,
what is measured is the time at which the rotor blade enters the
field of view and leaves the field of view of each of the two
sensors. Based on this information and using well-known geometric
equations, the track height of the rotor blade can be readily
determined in a well-known fashion. Furthermore, the actual blade
angular velocity can be calculated as well. The accuracy of blade
velocity calculations is determined, in large measure, by the
ability to accurately identify both leading and trailing blade
edges of the blades as they pass through a measurement space. One
of the problems with prior art optical systems is the inability of
the track sensors to properly respond to light level changes as the
blades enter and leave their fields of view. This problem is often
aggravated by differences in paint and/or paint erosion on the
several blades, which further tends to affect the accuracy of the
blade height calculations.
Radio frequency or microwave devices such as the well-known Doppler
or pulsed radar systems have commonly be(n used as locating and
tracking devices. However, known radar devices such as Doppler
radar depends on change in frequency and uses complex circuitry to
measure changes in transmit frequency. Pulsed radar typically can
be used effectively only when the distance between the source and
the target is greater than 100 meters and is therefor not suited
for use as a helicopter blade tracking device, where the tracking
device may have to be mounted approximately one meter from the
track of the rotor blades.
SUMMARY OF THE INVENTION
These and other problems of the prior art are overcome in
accordance with the present invention by means of tracking
apparatus comprising a radiating antenna having a predefined
antenna impedance and detection circuitry for detecting deviation
in antenna impedance resulting from movement of an object within
the field of radiation of the antenna. Advantageously, the antenna
may be positioned in close proximity to the projected path of a
moving object, e.g. at a distance of on the order of one meter or
less.
In accordance with one aspect of the invention, the tracking
apparatus is used in a system for tracking the several blades of a
helicopter rotor and the antenna is directed toward the path of
travel of the rotor blades. A signal source connected to the
antenna transmits radio frequency signals to the antenna, causing
the antenna to radiate an electromagnetic field in the path of the
helicopter rotor blades. Signal detection circuitry, connected to
the antenna, provides an output signal when an a change in antenna
impedance occurs due to the entry of a rotor blade in the field of
the antenna. A computation of distance of each blade from the
antenna is derived in a standard fashion to provide a measure of
tracking of the rotor blades.
In a specific embodiment of the invention, a radio frequency (RF)
generator connected to the antenna provides a continuous wave
unmodulated signal. As a moving blade enters the beam, it causes a
change in the antenna field impedance, resulting in a modulation of
the transmitted signal. The modulated signal is received by the
antenna and is detected and amplified by circuitry connected to the
antenna.
Advantageously, the tracking arrangement of the present invention
comprises a compact antenna structure consisting of printed circuit
boards which are readily and unobtrusively mounted.
In a particular embodiment of the invention, a tracking arrangement
comprises a pair of spaced apart antennas, each connected to a
circuit arrangement including a signal source and circuitry of the
present invention for detection of change in impedance, whereby
distance of a moving object from a predetermined location may be
computed from signals indicative of time of detection at the spaced
apart antennas.
In accordance with one aspect of the present invention, a signal
source transmits an RF signal to an antenna, directed toward the
path of an object to be detected, causing the antenna to radiate an
electromagnetic field in the direction of the path of the object.
The antenna preferably has a well predefined antenna impedance
resulting in a well defined return loss. As the object passes
through the electromagnetic field of the antenna, the impedance of
the antenna changes, resulting in a change in the return loss.
Effectively, movement of the object through the antenna field
causes an amplitude-modulation of the carrier signal. In addition
to causing a change in the amplitude of the return signal, the
moving object also causes a change in the phase of the return
signal with respect to the transmitted signal, resulting in a phase
modulation.
Advantageously, the blade tracking device in accordance with the
invention acts as a motion detector which is sensitive to change in
distance between the antenna and the moving objects.
Advantageously, the detector device of the present invention is not
sensitive to light and can be used in bright sunlight as well as in
darkness. Neither do clouds, nor fog, nor the color of the items
being tracked have any effect on the measurements obtained by the
tracking device of this invention. Furthermore, the antenna
detector device of the present invention is small and may be housed
in a standard radome capable of withstanding normal environmental
changes such as moisture, sand, dust, and dirt as well as vibration
and shock or exposure to manmade corrosives such as fuels and
lubricants.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a helicopter having a blade
tracking antenna mounted thereon;
FIG. 2 is a circuit diagram representation of an illustrative
embodiment of circuitry for the blade tracking device in accordance
with the invention;
FIG. 3 is a top elevational view of a printed circuit board
supporting an antenna in accordance with the invention for use with
the circuitry of FIG. 1;
FIG. 4 is a right side elevational view of the circuit board of
FIG. 3.
FIG. 5 is a plan view of a dual antenna structure for use in a dual
position tracking arrangement; and
FIG. 6 is a partial cut-away front elevational view of the antenna
structure of FIG. 5.
DETAILED DESCRIPTION
FIG. 1 shows a helicopter 50 having a fuselage 51 and a rotor 52.
The rotor is provided with a plurality of rotor blades 54, each
having a rotor tip 55. A radome antenna housing 60 containing a
blade tracker antenna for sensing the rotor blades 54 is mounted to
the fuselage 51 in an area of the fuselage preferably in
substantial alignment with the path followed by the tips 55 of
rotor blade 54 when the rotor is activated. Further, shown in FIG.
1 is tail rotor 59 and a radome antenna housing 61 containing a
blade tracker antenna for sensing the blades of the rotor 59. As is
described further later herein, the blade tracker antenna comprises
a flat plate antenna connected to signal transmission and signal
receiving circuitry. The antenna continuously transmits an RF
signal in the direction of the path of the rotor blades. Changes in
impedance of the antenna caused by a rotor blade passing through
the field of the antenna are detected by detection circuitry, not
shown in FIG. 1. The circuitry senses the impedance changes and
generates output signals that may be used to provide an indication
of tracking of the several rotor blades to a pilot, while the craft
is in flight.
Referring now to FIG. 1 and FIG. 2, FIG. 2 shows an antenna 100,
such as may be disposed within the radome 60, for detecting
movement of an object, such as one of the rotating helicopter
blades 54, and is preferably positioned in alignment with the path
of travel of the tips 55 of the rotor blades 54. Further shown in
FIG. 2 is a block diagram of electrical circuitry 150 connected to
the antenna 100. The circuit 150 is designed for generating and
transmitting a radio frequency (e.g., 1800 MHZ) signal to antenna
100 and includes a directional coupler such as a well-known RF
circulator 120. The circulator 120 is connected to the antenna 100
by a conductor 122 and is connected via a conductor 124 to a
standard RF source oscillator 126. The output impedance of the
circulator 120 presented to the oscillator 126 is preferably a near
perfect 50 Ohms, resulting in a return loss, in practice, of
approximately 40 dB below the power level transmitted by the
oscillator.
Any object that passes through the field of the antenna will
modulate the transmitted carrier. The modulation frequency will be
directly proportional to the rate at which the object enters and
leaves the antenna field and- the modulation amplitude will be
directly proportional to the distance between the antenna and the
object. A transfer function can be calibrated in a well-known
fashion to produce a corresponding curve. An appropriate equation
can readily derived from the calibration data and used in the
computer 146 to calculate blade distance from the antenna 100.
Furthermore, the detected modulation phase data can also be used to
calculate distance, in a known fashion. Known phase detectors
resolve phase down to one degree. For distances greater than one
wavelength, the amplitude of the received signal is used as an
indication of the number of full wavelength between the object,
e.g. a rotor blade, and the antenna. The phase signal can be used
to determine the distance represented by any partial
wavelength.
The circuit 152 is provided for detecting modulations in antenna
impedance as one of the rotor tips 55 passes through the field of
the antenna 100. An RF amplifier 128 is connected to circulator 120
via conductor 129. The amplitude signal to the RF amplifier 128 is
approximately 40 dB less than the transmit amplitude to the antenna
100. When an object, i.e., the tip of a rotor blade, passes through
the field of the antenna, causing a change in the field of the
antenna, a change in the return loss from the 40 dB level will
occur. When the object leaves the field, the return loss seen at
the oscillator 126 will return to the steady state level of
approximately 40 dB. The amplitude of the variation in return loss
is a function of the size of the object and of the material of
which the object is made, as well as a function of the strength of
the radiated field and the distance of the object from the antenna.
Additionally, the object causes a phase modulation of the return
signal as the object moves through the field of the antenna.
In the illustrative embodiment of FIG. 2, the circulator 120 is
further connected to a radio frequency (RF) amplifier circuit 128
via conductor 129. By operation of the circulator 120, a signal is
transmitted from the oscillator 126 to the antenna and from the
antenna to the RF amplifier 128. An amplified signal is transmitted
from the RF amplifier 128 via conductor 130 to a signal detector
132. The signal detector 132 provides an output signal only when
the input signal on conductor 130 has a signal level greater than a
predetermined threshold level. By proper selection of the
parameters of the amplifier 128 and the detector 132, a change in
antenna impedance caused by movement of an object, such as rotating
helicopter blade, in the vicinity of the antenna will be indicated
by an output signal from detector 132 on conductor 133. An
operational amplifier 134 connected to conductor 133 provides an
amplified detector output signal that is transmitted to a low pass
filter 138 via conductor 135. Operational amplifier 134 preferably
consists of operational amplifiers with a total gain on the order
of 50 dB. The RF amplifier 128 preferably has a minimum gain of
approximately 40 dB.
The low pass filter 138 serves to eliminate noise signals outside
of the radio frequency range and provides an input signal to an
analog circuit 140 via conductor 139. The analog circuit 140 may,
for example, be a well-known circuitry that provides a square wave
output pulse in response to each. signal from the filter 138
indicative of a rotor blade passing through the field of the
antenna. The width of the analog circuit output pulse is directly
proportional to the width of the rotor blade passing through the
antenna field and the amplitude of the analog circuit output pulse
will be directly proportional to the distance between the antenna
and the object. The analog circuit 140 is connected via conductor
145 to a logic circuit 146. The logic circuit 146 may be a computer
or other well-known circuitry, commonly used in helicopters and the
like, for computing a balancing solution. Logic circuit 146 is
responsive to output signals from the analog circuit 140 to provide
an output signal representative of the distance of the track of a
blade from a predefined level. The output of the logic circuit 146
may be used to compute a balancing solution for adjusting balancing
weights or trim tabs for the individual blades to improve tracking,
in a manner well-known to those skilled in the art.
Shown in FIGS. 3 through 6 is a dual antenna structure for use as a
blade tracker, such as may be contained in the radome 60, shown in
FIG. 1. The blade tracker may use such a dual antenna structure.
The methodology for measuring the distance of a moving object such
as a rotor blade from a specific point using two detector is well
known to those skilled in the art and has been described in prior
patents such as U.S. Pat. No. 3,023,317 issued Feb. 7, 1962, in the
name of S. P. Willits, et al. and U.S. Pat. No. 4,812,463 issued
Mar. 14, 1989, in the name of Richard Talbot. The noted prior art
patents relate to the use of optical devices to determine proper
tracking and describe methodology for determining distance from a
fixed point of objects moving at a known velocity. The method
involves the use of two spaced-apart detectors extending at a
predefined angle with respect to each other and directed to the
path of travel of the objects. Assuming that the rotor rotates at a
known speed, the distance of the several blades from the antenna
can be mathematically determined from the difference in time of
detection at the two separate detectors. The two antennas of FIGS.
3 through 6 radiate in the direction of the moving rotor blade or
the like at pre-defined angles to allow for the computation of the
distance of the track of the moving object from a fixed reference
point. The power required at the antenna is a function of the
approximate distance of the path the from the antenna. A transmit
power of approximately 50 milliwatts into the antenna has proven to
be effective at a distance of approximately 20 feet. The power
required, for example, at 4 feet has been found to be on the order
of 20 dB less than the transmit power at 20 feet. The transmit
frequency is a factor essentially only of the availability of the
components, costs and antenna size. In one particular embodiment,
the selected frequency is 1.803 GHz.
Antenna gain can be readily adjusted by selecting the antenna type
and size. In a practical helicopter installation, the size is a key
factor since the antenna is typically located external to the
helicopter fuselage. In one specific embodiment, two antennas are
directed such that the angle between the beams from the two
antennas is approximately 13 degrees. Antenna impedance directly
influences the system sensitivity, since motion is detected from an
amplitude modulation of the return signal resulting from changes in
the impedance of the field of the antenna. The antenna impedance is
preferably closely matched to 50 Ohms, ideally within 0.5 Ohms.
Sensitivity of the system is determined by a proper matching of
impedance and has a bearing on the range and transmit power
requirements of the system. The percentage of the amplitude
modulation of the return signal may be increased by increasing the
transmit power or increasing sensitivity and gain of the receiving
circuitry. An antenna bandwidth of 30 MHZ has found to be adequate.
The transmit frequency is preferably in the range of 1 GHz to 7
GHz. In a particular embodiment of the invention, the amplifier 128
has a noise figure of approximately 3.3 dB and a gain of
approximately 1.0 dB.
FIG. 3 is a plan view of a circuit board 200 supporting a blade
tracker antenna structure in accordance with the invention and
adapted to be mounted within the radome 60. FIG. 4 is a side
elevational view of the circuit board of FIG. 3. As shown in FIG.
4, a ground plane 208 is attached to the underside of the printed
circuit board 206. Further shown in FIG. 4 is a connector block
212, by which the center feed 104 is provided. The tuning capacitor
210 has a further connection to the ground plane 208 at 215, as
shown in FIG. 4. The antenna structure comprises two spaced apart
antennas in the form of copper layers 201, 202 disposed on the
printed circuit board 200. The copper layers 201, 202 each measure
approximately 1.5" square. An antenna feed connection 204 provides
a connection from two separate RF circulators 120 (shown in FIG. 2)
to the two separate antennas. The circulator transmits signals to
the associated of the antennas 201, 202, and receives signals from
one of the antennas 201, 202. The antennas 201, 202 are mounted on
a fiber glass printed circuit board 206. Further shown on the
circuit board 200 is a tuning capacitor 210 used to tune the two
antennas in a well-known fashion. Capacitor 210 is shown to be
connected to the copper layer 202 by conductor 211.
FIG. 5 is a plan view showing the antenna structure of FIGS. 3 and
4 in a support housing structure 250. FIG. 6 is partial cut-away
frontal elevational view of the structure of FIG. 5. The support
structure 250 has front and rear walls 251, 252 and side walls 253,
254. Brackets 256, 257 retain the two antennas 201, 202 on the
structure. As shown in FIG. 6, the two antennas are each disposed
at an angle A from vertical to aid in distance calculations as
described in the above-referenced U.S. patents. A preferred angle
is defined by the distance of the antenna from the plane of the
rotor and the desired spacing between two measurement locations in
the plane of the rotor. In one embodiment, the angle is
approximately 6.5 degrees from a line extending perpendicularly to
the plane of the rotor. It will be understood that the
above-described arrangement is merely illustrative of the
application of the principles of the invention and other
arrangements may be deemed by those skilled in the art without
departing from the scope of the invention as defined by the
appended claims.
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